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          大小端
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        <h2 id="1-定义"><a href="#1-定义" class="headerlink" title="1. 定义"></a>1. 定义</h2><ul>
<li><code>Little-Endian</code>:<code>低位字节排放在内存的低地址端，高位字节排放在内存的高地址端</code>。(逻辑上的低低高高)</li>
<li><code>Big-Endian</code>:<code>高位字节排放在内存的低地址端，低位字节排放在内存的高地址端</code>。(像数据流一样填充)</li>
<li>网络字节序：<em>TCP/IP各层协议将字节序定义为<code>Big-Endian</code></em>，因此TCP/IP协议中使用的字节序通常称之为网络字节序。</li>
</ul>
<h2 id="2-常见的CPU架构的字节序"><a href="#2-常见的CPU架构的字节序" class="headerlink" title="2. 常见的CPU架构的字节序"></a>2. 常见的CPU架构的字节序</h2><ul>
<li>Big Endian : PowerPC、IBM、Sun</li>
<li>Little Endian : x86、DEC</li>
<li>ARM : 大小端可选</li>
</ul>
<h2 id="3-判断方法"><a href="#3-判断方法" class="headerlink" title="3. 判断方法"></a>3. 判断方法</h2><p>大小端问题主要涉及的是<strong>非单字节非字符串外的其余数据</strong>的表示和传递，如short型、int型等。判断主机大小端的方法有很多，常见的是联合体判断法。</p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">bool</span> <span class="title">isBigEndian</span><span class="params">()</span> </span>&#123;</span><br><span class="line">    <span class="keyword">union</span> &#123;</span><br><span class="line">        <span class="keyword">int</span> a;</span><br><span class="line">        <span class="keyword">char</span> b;</span><br><span class="line">    &#125; num;</span><br><span class="line">    num.a = <span class="number">0x1234</span>;</span><br><span class="line">    <span class="keyword">return</span> (num.b == <span class="number">0x12</span>)</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h2 id="4-转换方法"><a href="#4-转换方法" class="headerlink" title="4. 转换方法"></a>4. 转换方法</h2><p>出于效率考虑，大小端的处理在客户端。在客户端socket过来时把服务器主机的大小端通知给客户端，这样服务器就不需要改动，直接传递数据就行，这时候可以在客户端代码中封装几个宏，在客户端在收到数据后，根据那些宏来判断是否转换以及得出转换后的数值。<br>大小端转换最有效也是最常见的方法就是移位法。</p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">#<span class="meta-keyword">define</span> __SWP16(A)   (( ((uint16)(A) &amp; 0xff00) &gt;&gt; 8)    | \  </span></span><br><span class="line">(( (uint16)(A) &amp; <span class="number">0x00ff</span>) &lt;&lt; <span class="number">8</span>)) <span class="meta">#<span class="meta-keyword">define</span> __SWP32(A) ((( (uint32)(A) &amp; 0xff000000)&gt;&gt; 24) | \</span></span><br><span class="line">(( (uint32)(A) &amp; <span class="number">0x00ff0000</span>) &gt;&gt; <span class="number">8</span>)   | \  </span><br><span class="line">(( (uint32)(A) &amp; <span class="number">0x0000ff00</span>) &lt;&lt; <span class="number">8</span>)   | \  </span><br><span class="line">(( (uint32)(A) &amp; <span class="number">0x000000ff</span>) &lt;&lt; <span class="number">24</span>))</span><br></pre></td></tr></table></figure>

<h2 id="5-为什么主机的字节序不统一"><a href="#5-为什么主机的字节序不统一" class="headerlink" title="5. 为什么主机的字节序不统一"></a>5. 为什么主机的字节序不统一</h2><p>这是因为 各个CPU厂商出于不同的逻辑考量，换句话说 大端和小端有其各自的优势。<br>我们知道计算机正常的内存增长方式是从低到高(当然栈不是)，取数据方式是从基址根据偏移找到他们的位置，从他们的存储方式可以看出，大端存储因为第一个字节就是高位，从而很容易知道它是正数还是负数，对于一些数值判断会很迅速。<br>而小端存储 第一个字节是它的低位，符号位在最后一个字节，这样在做数值四则运算时从低位每次取出相应字节运算，最后直到高位，并且最终把符号位刷新，这样的运算方式会更高效，也更符合我们手算的方式。</p>

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